1. Field of the Invention
The present invention relates to a connection structure of a circuit protection element, which can overcome an abnormal heat generation in a circuit protection element such as a positive characteristic thermister and its connection portion for cutting off overcurrent of an associated electronic circuit board.
2. Prior Art
In an electronic circuit board, a positive characteristic thermister (called PTC hereinafter) is widely used as a circuit protection element for cutting off overcurrent. The PTC has the character that its resistance increases in a quadratic curve with its temperature increase. The PTC generates heat to increase its resistance when provided with an overcurrent, which reduces or cuts off the current to protect the circuit.
Some conventional connection structures of a circuit protection element will be discussed hereafter.
FIG. 6 shows a first connection structure disclosed in Japanese Patent Application Laid-open No. H. 6-151107.
This structure is characterized in that PTC 41 contacts a spring piece 42 having a shape memory property. The spring piece 42 is connected to an electric power supply terminal 43. When the spring piece 42 is heated to more than a predetermined temperature, the spring piece 42 opens outwardly as shown by phantom lines to part from PTC 41, cutting off the power supply.
FIG. 7 shows a second connection structure disclosed in Japanese Patent Application Laid-open No. H. 8-250304.
The second structure characteristically has a pair of lead terminals 45, 46 connected to PTC 44 by solder 47, and one lead terminal 45 has an outward resilience to be spaced from the other 46. The lead terminals 45, 46 are connected to a circuit board 48. When PTC 44 generates an abnormal heat to melt the solder 47, the lead terminal 45 springs back to part from PTC 44, resulted in cutting-off of current.
Japanese Patent Application Laid-open No. H. 9-73848 discloses a connection structure having a coil spring (not shown) provided in line with the lead terminal 45 described of FIG. 7. The coil spring provides an increased spring-back force for the lead terminal. The lead terminal may be connected to the coil spring by soldering to cut off one another in the event of overcurrent.
However, the aforementioned first connection structure (FIG. 6) includes the spring piece 42 made of a shape-memory alloy which is expensive. The second connection structure (FIG. 7) must be connected to PTC 44 by soldering against a spring-back force of the lead terminal 45, which requires a troublesome connection work thereof The lead terminal having the coil spring also requires a complicated connection work thereof and increases in the part number/cost. These are disadvantage in the prior arts.